The growing political and moral corruption of Vienna is a great worry to its kindly, temperate ruler, Duke Vincentio. Knowing that he himself is as much to blame for the troubles as anyone because he has been lax in the enforcement of existing laws, the duke tries to devise a scheme to revive the old discipline of civic authority.
Fearing that reforms instituted by himself might seem too harsh for his people to accept without protest, he decides to appoint a deputy governor and to leave the country for a while. Angelo, a respected and intelligent city official, seems just the man for the job. The duke turns over the affairs of Vienna to Angelo for a certain length of time and appoints Escalus, a trustworthy old official, to be second in command. The duke then pretends to leave for Poland. In reality, he disguises himself as a friar and returns to the city to watch the outcome of Angelo’s reforms.
Angelo’s first act is to imprison Claudio, a young nobleman who got his betrothed, Juliet, pregnant. Under an old statute, now revived, Claudio’s offense is punishable by death. After being paraded through the streets in disgrace, the young man is sent to prison. He asks his rakish friend, Lucio, to go to the nunnery where Claudio’s sister, Isabella, is a young novice about to take her vows and to ask her to plead with the new governor for his release. At the same time, Escalus, who has known Claudio’s father well, begs Angelo not to execute the young man. The new deputy remains firm, however, in carrying out the duties of his office, and Claudio’s well-wishers are given no reason to hope for their friend’s release.
The duke, disguised as a friar, visits Juliet and learns that the young couple had been very much in love and had, in fact, been formally engaged; they would have been married but for the fact that Juliet’s dowry had become a matter of legal dispute. There is no question of heartless seduction in the case at all.
Isabella, going before Angelo to plead her brother’s cause, meets with little success at first, even though she has been thoroughly coached by the wily Lucio. Nevertheless, Angelo’s cold heart is somewhat touched by Isabella’s beauty. By the second interview, he has become so passionately aroused as to forget his reputation for saintly behavior. He tells Isabella frankly that she can obtain her brother’s release only by yielding herself to his lustful desires, otherwise Claudio will die. Isabella is shocked at these words from the deputy, but when she asserts that she will expose him in public, Angelo, amused, asks who will believe her story. At her wit’s end, Isabella rushes to the prison where she tells Claudio of Angelo’s disgraceful proposition. When he first hears the deputy’s proposal, Claudio is outraged, but the thought of death so terrifies him that he finally begs Isabella to placate Angelo and give herself to him. Isabella, horrified by her brother’s cowardly attitude, lashes out at him with a scornful speech, but she is interrupted by the disguised duke, who has overheard much of the conversation. He draws Isabella aside from her brother and tells her that she will be able to save Claudio without shaming herself.
The friar tells Isabella that five years earlier, Angelo had been betrothed to a high-born lady named Mariana. The marriage did not take place, however. After Mariana’s brother had been lost at sea with her dowry, Angelo broke the engagement, hinting at supposed dishonor in the young woman. The friar suggests that Isabella plan a rendezvous with Angelo in a dark, quiet place and then let Mariana act as her substitute. Angelo will be satisfied, Claudio released, Isabella still chaste, and Mariana provided with the means to force Angelo to marry her.
Everything goes as arranged, with Mariana taking Isabella’s place at the assignation. Cowardly Angelo, however, fearing public exposure, breaks his promise to release Claudio and instead orders the young man’s execution. Once again the good friar intervenes. He persuades the provost to hide Claudio and then to announce his death by sending Angelo the head of another prisoner who has died of natural causes.
On the day before the execution, a crowd gathers outside the prison. One of the group is Lucio, who accosts the disguised duke as he wanders down the street. Furtively, Lucio tells the friar that nothing like Claudio’s execution would take place if the duke had been ruler. Lucio confidentially says that the duke cares as much for the ladies as any other man and also drinks in private. In fact, says Lucio, the duke beds about as much as any man in Vienna. Amused, the friar protests against this gossip, but Lucio angrily asserts that every word is true.
To arouse Isabella to accuse Angelo publicly of wrongdoing, the duke allows her to believe that Claudio is dead. Then the duke sends letters to the deputy informing him that the royal party will arrive on the following day at the gates of Vienna and will expect to be welcomed. The command also orders that anyone who has grievances against the government while the duke is absent should be allowed to make public pronouncement of them at that time and place.
Angelo grows nervous upon receipt of these papers from the duke. The next day, however, he organizes a great crowd and a celebration of welcome at the gates of the city. At the prearranged time, Isabella and Mariana, heavily veiled, step forward to denounce Angelo. Isabella calls him a traitor and violator of virgins; Mariana claims that he will not admit her as his wife. The duke, pretending to be angry at these tirades against his deputy, orders the women to prison and asks that someone apprehend the rascally friar who has often been seen in their company.
Then the duke goes to his palace and quickly assumes his disguise as a friar. Appearing before the crowd at the gates, he criticizes the government of Vienna severely. Escalus, horrified at the fanatical comments of the friar, orders his arrest, seconded by Lucio, who maintains that the friar had told him only the day before that the duke is a drunkard and a frequenter of bawdy houses. At last, to display his own bravado, Lucio tears away the friar’s hood. When the friar stands revealed as Duke Vincentio, the crowd falls back in amazement.
Angelo realizes that his crimes will now be exposed, and he asks simply to be put to death without trial. The duke orders him to marry Mariana first, and he tells Mariana that Angelo’s goods, once they are legally hers, will secure her a better husband. The duke is surprised when she begs for Angelo’s pardon, in which entreaties she is joined by Isabella, but he relents. He does, however, send Lucio to prison. Claudio is released and married to Juliet. The duke himself asks Isabella for her hand.